The present invention relates to an illumination system designed to improve the detectability of some directional defects, such as bubble, in AOI system for detecting the defect in a transparent substrate, for example, a glass substrate.
In line scan AOI system, linear LED bar is normally used as illuminations. With the illumination of these linear LED bar, most defects could be detected by CCD camera. However, some directional defects, such as bubble, present great difference in signal intensity when the direction of defect is different. When a bubble is illuminated by linear LED bar and detected by AOI system, the detectability of bubble with its major axis setting along CCD scan line is very high while the detectability of bubble with its minor axis setting along CCD scan line could be quite low.
As shown in FIG. 1a, when bubble is set along with scan line with only one linear lighting set at one side of scan line, only part of bubble will be illuminated with strong enough intensity. The reason might be: (a) in one edge of bubble, lighting enters bubble and total reflection happened at air-glass interface so that most of the energy of lighting is reflected into CCD camera; (b) in another edge of bubble, lighting is mainly reflected by glass-air interface and limited energy is transmitted and finally received by CCD. While setting two lighting at both sides of scan line respectively, as shown in FIG. 1b, more part of bubble could be illuminated with strong enough intensity. In this case, most part of bubble is illuminated with strong enough intensity and bubble could be easily detected and classified.
If bubble is set perpendicular to scan line with one linear lighting, however, as shown in FIG. 1c, much less part of bubble could be illuminated with strong enough intensity. Even if two linear lightings are set at both sides of scan line, as shown in FIG. 1d, the light from bubble is still not strong enough for good detection and classification.
In area scan AOI system, area lighting such as round LED or rectangle LED is set as illumination. The detectability of bubble does not depend on the direction of bubble in such kind of system. The difference between area lighting and line lighting is obvious. By line illumination, bubble is illuminated by lighting from almost one direction, as shown in FIG. 2a. While by area illumination, bubble is illuminated by lighting from all directions, as shown in FIG. 2b. This difference could show the reason that detectability of bubble depends on its direction in line scan system.
However, it's difficult to adapt area lighting to line scan system due to the big dimension of some line scan system for the large-dimension transparent substrate. It's almost impossible to have area lighting with length in several meters.
Therefore, a novel illumination system for providing near isotropic illumination is designed to solve mentioned problem in this invention disclosure.